Use yfinance's new metadata feature to substantially improve performance

• Use of new metadata feature in yfinance massively speeds up price retrieval because the currency can now be retrieved via the history's metadata instead of additional calls to the Yahoo API.
• yfinance made timezones more accurate. Adjusted a number of tests accordingly.
• Fixes #13
• Fixes #14 (and similar issues)
• Requires update to yfinance >=v0.2.3